1. Field of the Invention
The present disclosure generally relates to image coding and, more particularly, to an image coding apparatus that compresses and codes a moving image, and a method thereof.
2. Description of the Related Art
Digital devices that can record a moving image, like a digital video camera and a hard disk recorder, have recently been prevalent. Such digital devices compress and code moving image data so that a moving image with a large amount of information is efficiently stored on a recording medium of limited capacity such as a flash memory and a hard disk. Typical compression coding schemes of a moving image include Motion Picture Experts Group (MPEG) and H.264. Such compression coding schemes of a moving image compress the amount of information (the amount of data) by utilizing high autocorrelation (i.e., high redundancy) that many video signals have. Among the redundancies of a video signal are time redundancy and two-dimensional (intra-picture) spatial redundancy. Time redundancy can be reduced by using block-by-block motion detection and motion compensation. Spatial redundancy can be reduced by using discrete cosine transform (DCT) and quantization. The compression and coding of a moving image further includes compression of the amount of data by using entropy coding.
The foregoing compression coding schemes of a moving image include the above-mentioned quantization of DCT coefficient data after DCT is performed. For the purpose of quantization, only one quantization step size is given to a macroblock of 16×16 pixels. Japanese Patent Application Laid-Open No. 2003-102007 discusses an apparatus in which a minimum value of a quantization step to give is determined frame by frame to efficiently assign code amounts to various types of moving images like ones with few motions and ones with a lot of motions.
In the field of moving image compression coding, a technique has been discussed that allows the quantized units of blocks to be variable, to further improve the coding efficiency. The technology discussed in Japanese Patent Application Laid-Open No. 2003-102007 is predicated on a condition that macroblocks (blocks to be quantized) to which a quantization step size is to be assigned have a fixed size. Therefore, the technology discussed in Japanese Patent Application Laid-Open No. 2003-102007 is not per se applicable to a compressing coding method for a moving image in which the units of blocks to be quantized are variable.